1. Field of the Invention
The present invention relates to optical transmission lines formed from optical fibers of differing mode fields. More preferably, the present invention relates to optical transmission lines having reduced splice loss.
2. Technical Background
Thermal expansion of an optical fiber mode field or xe2x80x9ccorexe2x80x9d to reduce splice loss or connector loss is known. For example, see: Hanafusa, H. and Horiguchi, M. xe2x80x9cThermally-Diffused Expanded Core Fibres for Low-Loss and Inexpensive Photonic Components,xe2x80x9d Electronics Letters, Vol. 27, No. 21 (10th October 1991); Shiraishi, K. et al., xe2x80x9cBeam Expanding Fiber using Thermal Diffusion of the Dopant,xe2x80x9d Journal of Lightwave Technology, Vol. 8, No. 8, pp. 1151-1161, 1990; Knudsen, S. et al., xe2x80x9cNew Dispersion Slope Managed Fiber Pairs for Undersea Fiber Optic Transmission Systems, 2001 SubOptics 2001 Conference T4.2.2; European Patent Application No. 1094346; and U.S. Patent Publication No. 2002/0159723.
Dispersion compensation or dispersion management has been used to control or compensate for chromatic dispersion, and/or dispersion slope, of an optical transmission line comprising optical fibers. Higher performance optical networks require a large number of splices between dispersion-managed optical fibers, such that the losses incurred because of optical fiber splices can become appreciable. Reducing the splice loss of optical fibers having dissimilar mode fields has been particularly difficult.
As the capabilities of optical communication systems expand, the reduction of splice loss at not only at a single wavelength but across a wavelength range, or ranges, becomes increasingly important. For example, in a long haul transmission line comprising a plurality of amplifiers such as erbium doped fiber amplifiers, losses that vary with wavelength get amplified by the series of amplifiers.
The connecting of two optical fibers by fusion splicing typically begins with removing the coating on each optical fiber at the respective adjacent ends, then the adjacent end faces of the two optical fibers are butted together, and the end faces are softened and fusion-spliced by heating with an are discharge or the like.
Disclosed herein is an optical fiber transmission line comprising a first optical fiber portion fused to a second optical fiber portion at a splice region, the first optical fiber portion having a first MFD at 1550 nm and the second optical fiber portion having a second MFD at 1550 nm, the first MFD differing from the second MFD, wherein the splice region has a splice loss less than 0.15 dB for all wavelengths between 1530 and 1570 nm. Thus, the optical fiber transmission line is loss-flattened. In preferred embodiments, the first MFD differs from the second MFD by more than 2 xcexcm at 1550 rum. In preferred embodiments, the first optical fiber portion has positive dispersion at 1550 nm, and the second optical fiber portion has negative dispersion at 1550 rum. In other preferred embodiments, the first optical fiber portion has positive dispersion from 1530 to 1570 run, and the second optical fiber portion has negative dispersion from 1530 to 1570 nm.
Also disclosed herein is a method of making an optical fiber transmission line such that the loss due to the spliced connection is reduced during the fabrication of the optical transmission line. In one preferred embodiment, the method comprises the steps of: (a) fusing a first optical fiber to a second optical fiber to form the optical fiber transmission line comprising first and second optical fiber portions joined at a fusion splice; (b) heating the optical fiber transmission line after step (a) with a flame directed at the fusion splice sufficient to grow the mode field of the first optical fiber portion and the second optical fiber portion at or near the fusion splice, wherein both the first and second optical fiber portions are heated substantially symmetrically about the fusion splice; and (c) offset heating the optical fiber transmission line after step (b) with a flame directed asymmetrically about the fusion splice, wherein the offset heating is sufficient to provide a splice loss less than 0.15 dB for all wavelengths between 1530 and 1570 nm.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.